An integrated circuit (IC) is usually constructed from a small chip of semiconductor material upon which an array of active/passive components have been constructed and connected together to form a functioning circuit. An IC is generally encapsulated in a plastic housing (chip) with signal, power supply, and control pins accessible for connection to external electronic circuitry. In some applications, an IC employs an input/output (I/O) circuit that is coupled to a selected signal pin where input signals are received and subsequently processed by an array of active/passive components in an input buffer. Also, output signals that were processed by the IC are presented at the same selected pin using an output stage that may include pre-driver and output driver elements. The output driver “pulls-up” or “pulls-down” a voltage level of the output in response to the signals generated by the array of components, thereby providing logic “1” and “0” signals which are transmitted to the external circuitry connected to the output. By adjusting the trip points of the pre-driver, different AC timing characteristics (delay and slew rate) of the output driver can be achieved.
A typical output driver includes a CMOS inverter with a p-channel pull-up transistor and an n-channel pull-down transistor. The n-channel and p-channel transistors in the CMOS inverter are often designed to supply enough current to a large load while the output swings between the rails of the power supply at a relatively high frequency. Also, circuitry external to the IC is usually employed to couple output drivers to a load condition.
Output drivers can be used to drive a signal onto a bus line that has a relatively large load. Output drivers are often designed and simulated to meet specific application requirements such as load size, as well as speed and/or noise compensation. Typically, output drivers are designed to quickly drive signals under expected load conditions, but at the same time, not too quickly so as to cause excessive noise problems. Additionally, a particular application of an output driver may be limited to a maximum slew rate so as to minimize undesirable high frequency components of the output signal and reduce unwanted radiation.
When the real world performance of an I/O circuit for an IC did not satisfy specification requirements, complete redesign of the circuit was often necessary. Historically, the trip points of an output stage or an input buffer were not adjustable after the IC was manufactured nor readily modified to compensate for a greater and/or different range of actual input/output loading conditions. Since an IC with non-conforming input/output performance is usually unfit for its intended purpose, it will be appreciated that enabling the performance of an I/O circuit to be tuned to meet specification requirements after the IC is reduced to silicon would reduce delays and costs associated with redesigning an I/O circuit for an IC manufactured for a particular use.